Lenses in general are used in many applications such as charged particle beam apparatuses which have many functions in a plurality of industrial fields, including, but not limited to, inspection of semiconductor devices during manufacturing, exposure systems for lithography, detecting devices and testing systems. A conventional lens is characterized by its focal length (or, as sometimes otherwise called, focal width or focal distance) and further by its optical axis. The focal length of lenses used in charged particle beam devices results essentially from the voltage or current applied which may be changed during operation. The optical axis may be understood as a main property of a lens used for charged particle beams such as an electron beam or an ion beam.
In many applications, the optical axes of the lenses applied coincide with the optical path of the beam. If the beam is used for analyzing a surface, for instance in a charged particle beam device, there are three techniques known in the state of the art to tilt the beam against the surface to be analyzed.
The first method is to tilt the specimen table in order to tilt the specimen fixed thereto. This way, the resulting angle between beam and specimen surface can be varied. However, there are also some disadvantages: The specimen table must be exactly tiltable; the specimen must be fixed to the table if the tilt angle exceeds a certain degree, otherwise it will slide; the focus of the beam has to be adjusted constantly if following up and down the tilted specimen; etc.
The second method is to leave the specimen table fixed but to tilt the charged particle beam device. That is, as in the previous case, a different angle between beam and surface results from mechanical tilting. The disadvantages are that there needs to be an exactly tiltable mechanical arrangement for tilting the device. Further, if the beam is tilted slightly further, the specimen plane is no longer in alignment with the optical width. Therefore, for every further tilting the focal width has to be adjusted again. This complicates the analysis of the specimen.
The third method is to leave both the specimen table and the beam device fixed. In this case, some deflection assembly is incorporated in the device in order to vary the angle between the beam and the specimen plane. There are, roughly spoken, the following three possibilities:
Firstly, a deflector is placed downstream of the objective lens. Thus, the beam can theoretically pass through the objective lens on its optical axis. However, the resolution suffers from the larger distance necessary between the objective lens and the specimen.
Secondly, a deflector is placed upstream of the objective lens. The deflected beam, however, passes the objective lens off-axially due to the deflection. Thus, this arrangement suffers from aberration.
Thirdly, at least two deflectors are placed upstream of the objective lens whereby the deflection orientation of at least two deflectors are opposite to one another. Hence, it is theoretically possible to deflect the beam such that it passes the objective lens in the middle. However, the beam still does not travel along the optical axis. This again may result in aberration. Furthermore, the optics is difficult to adjust.
It is accordingly an object of the present invention to provide a lens assembly for a charged particle beam device which overcomes at least some of the problems in the state of the art. Furthermore, it is an object of the present invention to provide a charged particle beam device which allows an improved three-dimensional analysis of a specimen surface. Furthermore, it is an object of the present invention to provide a method for operating a charged particle beam device that overcomes at least some of the problems in the state of the art.